Adrastea (, or as in Greek Αδράστεια), also known as , is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in Voyager 2 probe photographs taken in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than through telescopic photography. It was officially named after the mythological Adrastea, daughter of Greek god Zeus—the equivalent of Roman god Jupiter. Adrastea is one of the few moons in the Solar System known to orbit its planet in less than the length of that planet's day. It orbits at the edge of Jupiter's Main Ring and is thought to be the main contributor of material to the Rings of Jupiter. Despite observations made in the 1990s by the Galileo spacecraft, very little is known about the moon's physical characteristics outside its size and the fact that it is tidally locked to Jupiter.
Adrastea was discovered by David C. Jewitt and G. Edward Danielson in Voyager 2 probe photographs taken on July 8, 1979, and received the designation . Although it appeared only as a dot, it was the first moon to be discovered by an interplanetary spacecraft. Soon after its discovery, two other of the inner moons of Jupiter (Thebe and Metis) were observed in the images taken by a few weeks earlier by Voyager 1. The Galileo spacecraft was able to determine the moon's shape in 1998, but the images remain poor. In 1983, Adrastea was officially named after the Greek nymph Adrastea, the daughter of the Zeus and his lover Ananke.
No surface details of Adrastea are known, due to the low resolution of available images.
Due to tidal locking, Adrastea rotates synchronously with its orbital period, keeping one face always looking toward the planet. Its long axis is aligned towards Jupiter, this being the lowest energy configuration.
The orbit of Adrastea lies inside Jupiter's synchronous orbit radius (as does Metis’s), and as a result, tidal forces are slowly causing its orbit to decay so that it will one day impact Jupiter. If its density is similar to Amalthea's then its orbit would actually lie within the fluid Roche limit. However, since it is not breaking up, it must still lie outside its rigid Roche limit.
It seems that Adrastea is the most copious source of this ring material, as evidenced by the densest ring (the Main Ring) being located at and within Adrastea's orbit. More precisely, the orbit of Adrastea lies near the outer edge of Jupiter's Main Ring. The exact extent of visible ring material depends on the phase angle of the images: in forward-scattered light Adrastea is firmly outside the Main Ring, but in back-scattered light (which reveals much bigger particles) there appears to also be a narrow ringlet outside Adrastea's orbit.